The present invention relates to a hybrid vehicle having an engine and a motor-generator as a driving power source and, more particularly, to a device and method for preventing the motor-generator from being locked even under load operating conditions.
Various drive systems have been proposed for hybrid vehicles, which generally combine engines with motor-generators as driving power sources and switch between an electric drive (EV) mode in which the hybrid vehicles are powered only by the motor-generators and a hybrid drive (HEV) mode in which the hybrid vehicles are powered by both the engines and the motor-generators. Japanese Laid-Open Patent Publication No. 11-082260 discloses one such type of hybrid drive system that includes an engine, a transmission, a motor-generator with an energization control circuit (inverter), a first clutch disposed between the engine and the motor-generator and a second clutch disposed between the motor-generator and the transmission, so as to establish the EV mode by disengagement of the first clutch and by engagement of the second clutch and to establish the HEV mode by engagement of the first and second clutches. This hybrid drive system presents a possibility that the motor-generator gets overloaded and becomes unable to rotate properly even under load operating conditions, for example, when the hybrid vehicle makes a sudden uphill start in the EV mode. In such a motor locked state, the motor-generator energization control circuit sets up continuous energization only in its specific phase and causes a temperature increase of the motor-generator by heat generation. The hybrid vehicle becomes more incapable of proper start as the output torque of the motor-generator decreases with such a temperature increase.